This invention relates to a liquid crystal device and, more particularly, to a liquid crystal device suited to be driven for switching by thin film transistors.
FIG. 1 is a sectional view showing an example of a prior art liquid crystal device with an array of thin film transistors (TFT) for one picture element. On a base plate 2 of glass or the like are successively provided a gate electrode 8, an insulating film 7 and a semiconductor layer 6. Further, a source layer (i.e., signal line) 5 and a drain layer 4 are provided on the gate electrode 8, with the drain 4 connected to a picture element electrode 3 provided on the base plate 2. A liquid crystal orientation film 9 is provided on the entire surface of the structure described above. On another base plate 1 are provided a picture element electrode 3 and an orientation film 9. The two base plates noted above are arranged so that they face each other, and they are bonded to each other at their peripheral parts by an epoxy adhesive or the like. A liquid crystal 11 is sealed in the space thus defined between the two base plates.
FIG. 2 shows an equivalent circuit of the liquid crystal device described above. Designated at R.sub.ON and R.sub.OFF are resistances of the TFT in the "on" and "off" states, respectively, at C.sub.i1, C.sub.i2 and C.sub.LC capacitances of the orientation films and liquid crystal, respectively, and R.sub.i1, R.sub.i2 and R.sub.LC are resistances thereof, respectively.
FIG. 3 shows an example of voltage waveforms when the TFT is driven. It is assumed that a gate pulse as shown at V.sub.G in FIG. 3 is applied to the gate electrode 8 shown in FIG. 1 and a voltage of +V.sub.S volts is simultaneously applied to the source 5. At this time, the voltage V.sub.LCD between the upper and lower picture element electrodes 3 quickly rises to +V (charge process). With a subsequent fall of V.sub.G the voltage V.sub.LCD is reduced from +V.sub.S due to leakage of charge stored in the liquid crystal layer (discharge process). The time constant .tau..sub.OFF in the discharge process is given as EQU .tau..sub.OFF =R.sub.LC .multidot.C.sub.LC ( 1)
As is seen, it depends on the resistance R.sub.LC of the liquid crystal layer.
The effective voltage applied to the liquid crystal layer is shown shaded in FIG. 3. At instant t =t.sub.1, it is shown as V.sub.LC (0). Shown at V.sub.i is a voltage applied across the orientation films and not across the liquid crystal layer.
The voltage is given as ##EQU1## where ##EQU2##
With reducing resistance R.sub.LC of the liquid crystal layer, the shaded area in FIG. 3 is reduced, i.e., the effective voltage across the liquid crystal layer is reduced. Therefore, with a horizontally orientated liquid crystal device (LCD) using a nematic phase liquid crystal having a positive dielectric anisotropy, the V (voltage) versus T (transparency) curve obtained when the LCD is TFT-driven is shifted toward the higher voltage side when compared with the curve obtained by driving the LCD with a rectangular pulse or wave. A curve (a) in FIG. 5 (or FIG. 8) is obtained with an LCD using "E-7" (a biphenyl type nematic liquid crystal composition available from BDH Chemical Ltd.). The LCD has 1,000 .ANG. polyimide orientation films and has a cell gap of 10 .mu. and a twist angle of 90.degree. .
FIG. 5 (or FIG. 8) shows examples of the waveform of V.sub.LCD when V.sub.S =6 V. A liquid crystal device for a television set or the like is usually of a line sequential drive type for display on screen. Therefore, it is necessary to hold information on each scanning line while it is not selected. In the case of 60 Hz non-interlace mode, this period is approximately 16.7 msec. If V.sub.LC (t) becomes lower than the threshold voltage V.sub.th of the liquid crystal, the image can no longer be displayed. Where "E-7" is used under the ordinary conditions, V.sub.LCD is greatly reduced as shown by a curve (a) in FIG. 5 (or a curve (a) in FIG. 8). This is attributable to the fact that the resistance R.sub.LC of the liquid crystal layer is low.
As has been shown, when the resistance of the liquid crystal is reduced in case of the TFT driving of LCD,
(a) the voltage versus transparency (V-T) curve is shifted toward the higher voltage side, i.e., the drive voltage is increased, and
(b) no image appears on the screen in an extreme case.